Many structural components which traditionally have been fabricated from metals are being replaced by their polymeric counterparts. For example, unidirectionally reinforced graphite fiber/epoxy composite is often used in aerospace structural applications. The main advantage of using polymeric materials lies in the ease of production of complicated parts, simple assembly, fewer parts, and fewer finishing operations. Another major advantage, in aerospace applications, is the lower density of polymer composites as. compared to metal, which results in higher specific properties (i.e., performance per pound of material) for polymer composites.
The use of thermoplastic matrix composites for aircraft structures offers advantages over the more conventional epoxy composites, including reduced sensitivity to moisture effects and improved impact performance.
A highly preferred thermoplastic resin is PEEK. This material, a polyether etherketone resin, when reinforced with approximately 60% carbon fiber, yields a composite material stronger and lighter than many aluminum alloys.
Polyether etherketone (PEEK) resin/carbon fiber (CF) composites are, accordingly, being used in wear resistant applications, including aerospace applications. Continuous fiber reinforced pre-preg can be fabricated into tubes, which can be sectioned and machined into bearings, washers, wear rings, bushings, etc., for use in pumps, centrifuges, compressors, and similar equipment. The PEEK/CF-type composites offer other advantages over other materials, including: the ability to tighten clearances, and hence improve efficiencies of pumping; good wear resistance even at high temperatures; resistance to aggressive environments; and the ability to run dry without catastrophic failure.
A number of problems with thermoplastic polymers have precluded their being used more extensively as a replacement for more conventional metals. One of the primary concerns in the use of polymer composites is the time dependent response of polymeric materials. So-called "creep deformation" is significant in many polymers, even at room temperature, and is rapidly accelerated by small increases in temperature. A 50.degree. C. rise in temperature above room temperature could have a catastrophic effect on the life of polymeric parts, but little effect on their metallic counterparts. Creep response is also affected by the physical aging of polymers, a phenomenon where the polymer exhibits volume and enthalpy relaxation with time even at temperatures below the glass-rubber transition temperature (T.sub.g) of the polymer.
The molecular structure of thermoplastic polymers is different from that of traditional materials, such as metals, in that the polymeric macromolecules are held together by weaker secondary bonds as opposed to the strong metallic bonds in metals. Consequently, the conformation of the molecules changes continually as a result of the thermal energy contained in the system. When subjected to an external stress, rearrangements on a local scale take place fairly rapidly but those on a larger scale occur rather slowly. These long-scale rearrangements are referred to as the "creep" response. The distribution in the molecular weight of the polymers leads to a wide range of time scale over which the "creep" behavior of polymers is observed. The term "time-dependent" behavior is often used synonymously for "creep" behavior. The term, however, has a more general connotation and is also used to describe the "stress-relaxation" response of polymers.
In certain circumstances, thermoplastic composites are known to undergo a permanent, irrevocable deformation when held under constant load, a phenomenon which is also usually referred to as "creep." This phenomenon is well documented. See, e.g.:
A. Horoschenkoff, J. Brandt, J. Warnecke and O. S. Bruller "Creep Behaviour of Carbon Fibre Reinforced Polyetheretherketone and Epoxy Resin" SAMPE Conference, Milan "New Generation Materials and Processes" 339-349 (1988);
C. Hiel "Creep and Creep Recovery of a Thermoplastic Resin and Composite" Proc. AM. Soc. for composites 3rd Technical Conf. 558-563 Technomic Publishing (1988);
D. H. Nguyen, S. F. Wang and A. A. Ogale "Compressive and Flexural Creep Deformation in Thermoplastic Composites" 34th International SAMPE Symposium 1275-1282 (1989); and
A. A. Ogale "Creep Behaviour of Thermoplastic Composites" in `Thermoplastic Composite Materials`, Ed. L A Carlsson; Elsevier (1991),
all of which are incorporated in their entireties by reference herein.
In many applications it is necessary to "interference fit" the thermoplastic composite bearing or other cylindrical member onto a metal shaft in order to retain it in location; this interference fit must be retained over the operating temperature of the equipment. Users of such products have identified a problem in which PEEK/CF wear rings, for example, when interference fitted onto a shaft at room temperature and subsequently run at temperatures of up to 450.degree. F. subseqeuntly lose the interference fit at room temperature, resulting in the ring moving along the shaft due to the differential pressure across the pump, and a consequent loss in efficiency of the pump.
The existence of a creep deformation in the mode required to cause an increase in inner diameter of a thermoplastic cylinder, however, is unexpected. There appears to be no information available in the prior art directly pertaining to the phenomenon in thermoplastics, and there is little, if any, information regarding this creep phenomenon in other composite materials.
Accordingly, it would be desirable to provide a method of preconditioning thermoplastic cylindrical members fitted on shafts to avoid the irrecoverable deformation or "creep" which plagues the existing state of the art.